Low frequency loudspeaker



Oct. 15, 1957 P. J. BROWNSCOMBE 2,810,021

LOW FREQUENCY LOUDSPEAKER Filed Jan. 5, 1955 5 Sheets-Sheet l 1957 P. J. BROWNSCOMBE 2,810,021

LOW FREQUENCY LOUDSPEAKER Filed Jan. 5, 1955 s SheetsSheet 2 PHHJP J. BROWNSCQMBE 1957 v P. J. BROWNSCOM BE 2,810,021

LOW FREQUENCY LOUDSPEAKER 5 Sheets-Sheet 3 Filed Jan. 5, 1955 PHIL \P J. Braowmscomstl United States PatentO 2,810,021 Low FREQUENCY LOUDSPEAKER Philip J. Brownscombe, Chatham, N. J. Application January 5, 1955, Serial No. 479,902

17 Claims. (Cl. 179-1155) The invention relates to loudspeakers such as are in 1 corporated in the construction of radio and television re; ceivers and record players and has particular reference to loudspeakers used for the reproduction of low frequency sound in conjunction with other speakers ,which reproduce the higher frequencies. 7

Multiple speaker systems which combine low frequency I production of the very low frequencies has not kept pace,

so many original inventions and improvements are currently directed to that end. Many quality amplifiers now in use will operate efficiently with low distortion and high power output down to the to cycle range, or even lower, and yet no loudspeaker can be claimed to be efiicient in operation at 20 cycles and it is well known that present day speaker systems have high distortion and low power handling capacity at theirlower limits of operation. The fundamental difficulty in reproducing the extreme low frequencies, as, in bass notes of the musical scale, is that a very large volume of air must be displaced by the speaker mechanism and the efiiciency of a transducer falls off rapidly (inversely as the 4th power of frequency in the case of a direct radiator) below its fundamental resonant frequency. This requires, then, that low frequency speakers, known to the trade as woofers, shall have a low resonant frequency and a large volume displacement; the latter signifying large area and excursion of the diaphragm.

It is present day practice to employ-various means for increasing the sound energy radiated by a speaker at the low frequencies. Multiple speakers are used to increase diaphragm area, as previously stated, and resonant cavities are sometimes used. More commonly, some sort of horn is employed. The horn serves to increase the volume of air displaced at the mouth as compared to that displaced at the throat with a corresponding difference in air pressure at the horn ends. The horn is very effective but tends to be exceedingly and objectionably bulky. Because of thisfact, one inventor has attempted to reduce the room space occupied by a horn by incorporating .a folded horn in the internal structure of the cabinet of a reproducing apparatus and by further providing for location of the cabinet in a corner of 'the room in such a mafiner that the cabinet sides combine with the adjacent walls of the room to form an extension of the folded horn concealed within the cabinet. In spite of the help of these horn devices, however, there still remains a serious problem to be solved in order to obtain sufficient sound output at low frequencies. For example, when one side of the speakerradiates directly into the air and the other through a folded horn, at the long wave lengths corresponding to low frequencies, the front radiation subtracts from the output of the'horn. If, instead of using a horn, a speaker of large diaphragm area is mounted so that one side radiates into the openair and the other side radiates into a completely enclosed space, in the manner heretofore commonly practiced, a prohibitively large enclosure is necessary or the resonant frequency of the diaphragm will be come objectionably highowing to the stiffness of the enclosed air' volume. i

Stiffness may be defined as that property of a mechanical member or system which results in a force opposing deflection of the member, said force being a function of the displacement. The last phrase is necessary to distinguish the property from. resistance and in; er'tia, which exhibit forces that are functions of velocity and acceleration, respectively. $titfness is the fundamental property of asp ring, Its numerical value is the force necessary to deflect the spring one unit of length. Compliancefwhich is another expression for the same phenomenon, is the reciprocalof stiffness.

Stiffness is such a common and basic property of mechanical devices that it is commonly denoted by a symbol for convenience in reference and in computation. For example, inthe case of the very simple system consisting of a spring connected between a mass and a fixed body, the resonant frequency is given by the formula:

k=stiifness nse Stiffnesses maybe combined in series or in parallel. If in parallel, the stiffnesses add numerically; if in series, the compliances add.

Stiffness in a mechanical system is equivalent to a condenser in an electrical system. More strictly, mechanical compliance is analogous to electrical capacitance. With proper substitution of inductance for mass, capacitance for compliance, and electrical resistance for viscous mechanical resistance, electriccircuit theory and equations may be used to solve behavior problems in mechanical systems.

Concerning the conception of my present invention of negative stiffness, I had observed in my work that in some cases mechanical "stiffness is undesirable and that it would be useful to reduce the numerical value of this property where it is impracticable to remove the members which contribute the undesi ed stiffness. To this end, I

l 2 Where "2r m devised the mechanical assembly disclosed herein, whichhas the property, like a stiffness, of producing a force which is a function of the member displacement but, Iunlike the well known stiffness, the force produced is in the opposite direction; that is, it is in the direction of the displacement. I call this property negative stiffness? because it is the exact negative of the commonly known stiffness. For example, if a common stiffness of value A and a negative stiffness of absolute value B are put in parallel connection, as two springs would be connected to obtain stiffness A-l-B, the net result is a stiffness'of value A B. Therefore, this new device is mathematically equivalent to a stiffness ofneg ltive sign and may be so treated in the equations used ,tosolve problems involving stilfness. t

As previously intimated, aclose analogy is found in the electrical art where resistancehas been known and dc I fined for many years. More recently it has been observed that it is possible .to create an electrical system which, when added to a common resistance, results ina lower resistance for the combination. The property of the new system is such that problems involving it may besolved by considering it as aresistance with negative sign. This phenomenon now knownin the art as a, negative resistance although the ,cornrnonresistan-ce ,was' not, and still is not, called positive resistance.

Similarly, having applied to a mechanical system an assembly which exhibits a property equal to, but opposite in sign to, a common stifiness, I call it a negative stiffness, feeling that not only will this term be appropriate, but that the name itself will convey to those skilled in the art an understanding of the phenomenon being described. The term positive stiffness, which might be called ordinary stiffness, is used where there is a possibility of confusing the two.

The air volume stiffness on either side of the diaphragm in a sound reproduction apparatus acts exactly like a spring in opposing free movement of the transducer diaphragm. In other words, it is a force, such as that sup plied by any common spring as a function of the displacement thereof, that acts in a direction to oppose the displacement of the diaphragm.

Having in mind all the above enumerated difficulties experienced in previous attempts to achieve high fidelity reproduction of low frequency sounds, it is my primary object to solve the attendant problems by proceeding along a line of invention that may avoid the necessity for any horns or resonant cavities. To be more explicit, it is my purpose to build a low frequency speaker of the direct radiator type which is mounted in the wall of a small enclosed air chamber. The diaphragm area will be comparatively large and the amplitude as well, in order that considerable power may be radiated at low frequencies. Since, as previously explained, stiffness of the enclosed air volume in the proposed speaker construction will raise the resonant frequency of the diaphragm to an objectionable degree unless counteracted in some way, it is an ancillary object of the present invention to provide means for exerting a force, which may be termed negative stiffness to distinguish from the common, or positive, stiffness, on the diaphragm in a direction to increase its operational displacement as caused by the conventional drive means associated with the speaker instead of in opposition to displacement.

Briefly described, the specific means which I propose to employ for applying negative stiffness to a transducer diaphragm to insure full operational displacement is a toggle mechanism including spring-loaded pivoted struts which will be in a state of unstable equilibrium in axial alignment when the diaphragm, to which they are connected, is in its undisplaced neutral position, but which will be actuated by axial displacement of the diaphragm in either direction so that the spring-loading means will exert a component of its total force in the same direction as the diaphragm moves to increase its displacement and thereby negate the positive stiffness which the compression of the enclosed air volume tends to set up.

Since it is impracticable to perfectly seal the totally enclosed air chamber that I propose to use against leaks or to insulate it against thermally induced internal pressure changes, the toggle-type device for exerting negative stiffness on the diaphragm would be very likely to lock the latter in an excessively displaced position when it has been prevented from returning to neutral position by an unbalanced condition of pressure between the enclosed air and the outside atmosphere. It, therefore, is a further object of the invention to provide means for automatically maintaining the pressure of the enclosed air volume at a substantially constant value whereby the diaphragm will always return to neutral position following abnormal displacement in either direction.

Other objects, advantages and features of the invention will become apparent as the following specific description is read in connection with the accompanying drawings, in which:

Figure 1 is a vertical sectional view of a low frequency loudspeaker constructed in accordance with the present invention, showing the toggle struts in their dead-center state of unstable equilibrium corresponding to the neutral position of the loudspeaker diaphragm; and Figure 2 is a fragmentary side elevational view of the toggle mechanism alone, showing the displaced arrangement of the A toggle struts in one direction in full lines and the opposite displaced arrangement in broken lines.

Figure 3 is a diagram representing the resolution of forces during displacement of the toggle struts in one direction from dead-center position.

Figure 4 is a fragmentary side elevation of a modified embodiment of toggle mechanism; Figure 5 is a similar view of another modification of the same; and Figure 6 is a similar view of a still further modification.

Figure 7 is a schematic view of a pitot-tube type aircraft airspeed meter with the negative stiffness device of the present invention applied thereto to negate positive stiffness to which the sylphon bellows diaphragm is subjected.

Referring now in detail to the drawings, wherein like reference characters designate corresponding parts in the several views, it will be observed that the improved low frequency loud-speaker represented in its entirety in Figure 1 comprises three principal units or assemblies, which are the speaker proper A, the negative stiffness unit B, and the pressure equalization unit C.

Referring further to Figure l, the housing 1f shown therein serves as the enclosure for a dead air space and as means for mounting a direct radiator type of loudspeaker, which consists of a conical paper or composition diaphragm 11, an electrical winding or voice coil 12 rigidly attached to the throat of the cone, a magnetic struc ture 13 which causes voice coil 12 to operate in a magnetic field of high flux density, an annular centering member 14 which serves to keep the voice coil centered radially within the magnetic structure gap, and a pliable surround 15 formed integral with the mouth of the conical diaphragm 11 which centers the cone edge and permits free axial diaphragm movement under the influence of the drive means therefor constituted by voice coil 12 and magnetic structure 13. The loudspeaker organization just described may be of any conventional type or construction which usually includes a concavo-convex spider frame 16 to support the same in an installation opening in the front wall of housing 10 in closing relation thereto. The housing 10 should be substantially airtight, but the pressure equalization unit C, which will be described in detail later herein, may be depended upon to compensate for any air leaks that cannot be avoided.

Obviously, the magnetic flux for voice coil 12 may be produced by an electro-magnet instead of the perma nent magnet 13 shown, at the option of the designer. Moreover, other design features may be changed to suit the new mode of operation. For instance, it may be observed that the proportions of some parts are somewhat different from those in common use in that I plan to provide for greater axial movement of the diaphragm in order to be able to radiate large amounts of low fre quency sound.

Up to this point in the disclosure, it would appear that the arrangement and proportions of parts is deficient in that radiation or" low frequency sound requires a large area diaphragm and it has heretofore been under stood that a large area diaphragm requires a very large enclosure in order to keep the resonant frequency down. It is in these respects that I am enabled to depart from the earlier conventional practices and produce a high quality low frequency loudspeaker of greatly reduced over-all size, for the reason that the negative stiffness unit B which I have devised permits use of a small air chamber.

The negative stiffness unit B, in one embodiment thereof, is shown in Figure l as being mounted on the front of housing It This unit includes a rigid spider-type frame 17 of suitable form adapted to its purpose which is shown detachably secured to housing it) as by means of the same bolts 17' that fasten spider frame 16 of speaker A to the inside of said housing. Due to its open structure, frame 17 will not interfere with free sound radiation into the atmosphere by diaphragm 11.

Frame" 17 is provided with a central aperturels arranged" concentric to the axis of loudspeaker diaphragm 11' for reception of a reciprocating connecting rod 19, which may be of the solid construction shown, or tubular if desired (not shown)'and which should be light in weight. The rear end of connecting rod 19 is attached to the throat of diaphragm 11 by means of a'rigid member 20-20 which may be an impervious cone as shown, but it is within the spirit of the invention to employ any other suitable-attaching means for this purpose. A pair of rigid toggle struts 21-21 is located at the rear end of connectingrod 19. Both struts are arr'angdin generally axial alignment with each other in the-vertical plane of the common connecting-roddi'aph'ra'gm axis and are supported by means now to be described. The inner ends of struts 21-21 are tapered to form knife edge bearings 22-22 pivotally engaged in straight'transverseV-shaped bearing grooves 23-23 in "an enlarged portion 24- of connecting rod 19. The outer ends of struts 21-21 have similar knife edge bearings 25-25 engaged in transverse bearing grooves 26-26 provided in the inner sides of parallel light levers 27-27 near the forward ends thereof. These levers 27-27 are positioned respectively above and below connecting rod 19 substantially in the vertical plane thereof and have transverse bearing grooves 28-28 in their inner sides near the forward ends thereof for engagement by knife edge bearings 29-29 provided on the central portion of frame 17. To complete the operative mounting means for the toggle struts 21-21 and connecting rod 19, spring means of any suitable type, such as the coil springs 30-30 shown by way of example, are interposed between frame 17 and the medial portions of levers 27-27 respectively.

The length of strut loading springs 30-30 preferably should be such that they will reach from the frame 17 radially inward into close proximity to connecting rod 19 when in full extension and relaxed condition. As a result, springs 30-30 are capable of pressing the outer ends of struts 21-2-1 inward until the" struts become parallelwith' the connecting rod axis, although such a degree of displacement normally will never occur. In fact, to prevent-any undue displacement of the connecting rod and struts, stops 31 and 32 are provided on connecting rod 19 in suitably spaced relation for limiting contact with frame 27 in theregion of its central aperture 18.

The operation of negative stiffness unit B in'cooperation with loudspeaker A wilt-now be described.

It is to be understood that the unit is designed so that togglestruts 21-21 will be in axial alignment and thus in dead-center relation, as shown in Figure 1, when diaphragm 11- isin its mid-position, neutral state of rest. In this position, struts 21-21 will besubjected to maximuminward forceby spring-loading means 30-30 and therefore in astateo-funstable equilibrium. The slightest axial displacement of connecting rod 19 due to like displacement of'diaph'ragm 11, to the left for example, will cause the struts to move so that their axes are no longer parallel and coincident but intersect atan angle to the axis of rod-19as shown in Figure'Z. In this view, the plane of equilibrium is indicated by dot-and-dash line e-e, whereas the plane of displacement of the inner ends ofstruts 21-21- to the left is indicated by similar line tld". The plane of displacement in the opposite "direction is indicated by line d-d. The degree of displacement represented'in Figure 2 is greatly exaggerated forth'e purpose 'of illustration. Actually, the only time-that any such displacement could occur in either direction from the dead-center position of the togglest-ruts would be 'under the abnormal condidons-described later he'rein.

The force exerted upon: struts 21-21 throughlev'ers component axial with respect to rod 19, to .th'eleft, which force, for small displacements, is proportional to the displacement and in a direction tending-to increase the displacement.- Therefore, this force may bemade equal (but opposite in direction) to, or in any desired ratio to, the force produced on diaphragm 11 by the air pressure within the enclosure of housing 10, which pressure is caused by reduction in air volume resulting from a displacement of the diaphragm inward in compressing relation to said volume. relationship, the displacement of diaphragm 11 will be increased to such a degree that the fundamentalresonant frequency of the diaphragm may be made as low as desired.

The toggle mechanism, which has just been described briefly as to construction and general modeof ope'r ation in its adaptive capacity to negate the ositive stiffness opposed by the enclosed air volume to operational displacement of the speaker diaphragm, has further peculiarly useful and very important advantages which may be revealed bymore detailed discussion. For example, the arrangement of all moving. parts is symmetrical in relation to the common axis of connecting rod 19 and diaphragm 11, thereby serving to keep the motion imparted to the diaphragm in a straight line and'to keep the friction low; and, most important of all, it has been discovered that the toggle mechanism can be designed to afford a theoretically perfect linear force-displacement relationship even at large displacements.

Linearity is achieved as follows. The vertical spring force applied at the outer end of each toggle strut, by the fundamental nature of ordinary springs, is proportional to the displacement of the spring supplying this force. It the spring displacement measured at the outer strut end is made equal to the effective strut length, that is'from pivot to pivot, then the horizontal component of the force may be shown'to be exactly proportional to the horizontal displacement. Referring to the diagram represented in Figure 3, it will be supposed that a vertical downward force F is applied to the upper end pivot of the upper strut, whose lower end is displaced from the vertical by an amount X. The strut length is L and the vertical height under these conditions is H.

By resolution of forces, a compressive force F exists along the strut and exerts a horizontal component Frr at each end, the relationship being ELLE F H whence In this case then, the horizontal force is proportional to the horizontal displacement and the unit acts as a per This linear relationshipfectly linear negative stiifness. isdmportant in many applications because the positive stiffness which it is desired to counteract is usually linear.

The toggle unit therefore may be analyzed from another point of view as being a mechanical assembly possessing the properties of a linear negative stiffness spring conslsting of:

(a) a toggle strut pivoted in the middle (both struts 21-21 combined as one unitary structure);

([1) spring means applying equal compressive forces: to the unitary toggle strut'at both ends, which forces areapplied substantially in the direction'of the unbent strut; and

Due to this toggle mechanism (c) the maximum spring deflection being equal to the toggle strut length.

The speaker disclosed in Figure 1 would be completely satisfactory in operation provided that the enclosure were entirely free from leaks and that the enclosed air volume could be limited to small temperature and atmospheric pressure changes. None of these conditions exists in practicable construction or applications. Therefore, it is necessary to provide some means for keeping diaphragm 11 centered in its axial path of cyclic excursions, except. of course, when vibrating in response to sound waves. For example, suppose that, when equal pressure exists within and outside the enclosure, the toggle structure of the negative stiffness device B is not exactly centered, due to temperature-eifected displacement of the neutral center position of diaphragm 11. There is then a steady force exerted by the toggle mechanism upon the diaphragm which makes the pressure Within the enclosure greater or less than that outside. If there is any leakage at all, the diaphragm will gradually yield to the force and, as it does so, the force will increase until the diaphragm reaches its -maximum displacement as limited by one of the stops 31-32, where it will stay, unfit for use.

in order to achieve satisfactory lightness with required rigidity, conical diaphragms usually are made of paper and therefore are subject to slow gas diffusion under pressure. Even if the diaphragm, surround and enclosure could be made completely leakproof, changes in temperature or atmospheric pressure would cause intolerable displacement of the diaphragm. Therefore, it is necessary to provide some means for supplying a force which will keep the average position of the diaphragm near center while permitting it to vibrate at full amplitude at sound frequencies. This I have accomplished in my speaker by means of two small air pumps 33 and 34 which preferably are electrically operated. These pumps are diagrammatically represented in Figure 1 because their precise type and construction is not material to the claimed scope of the present invention, except that they should be quiet in operation so as not to aifect sound reproduction by diaphragm 11. The small, completely silent pumps commonly used for aerating aquaria would be particularly suitable.

Pump 33 when in operation withdraws air from the enclosure and forces it outside. This results in an atmospheric force pressing diaphragm I1 inward. Pump 34 on the contrary when in operation draws air from the atmosphere "into the enclosure and forces diaphragm ll outward. Since the pumps chosen for the purpose of illustration are electrical in general type, one side of the electrical circuit of each is connected permanently to ground, i. e. the housing of the loudspeaker housing lit, which should be made of metal for this purpose. The opposite side of the circuit of each pump is connected to a control switch 35 operatively associated with negative stiifness unit B. This switch 35 includes a body 36 of conductive metal, which should have considerable mass for a reason to be explained presently. Body 36 preferably is annular in structure and is supported flexibly in encircling relation to connecting rod adjacent to the central portion of frame 17 by spring means 37, which is represented as being a coil spring but may be of any desired construction. Body 35 is further supported by a coil spring 38, which is located on the opposite side thereof with respect to spring means 37 and balances the longitudinal thrust of the latter. Spring 38 is arranged concentric to connecting rod 19 and has its rear end seated preferably in a cupped disk 39 encircling said rod and bearing against the enlarged portion 24 thereof. A movable electrical contact 4%) carried by body 36 and consequently grounded electrically in the frame is normally maintained midway between insulator-supported stationary contacts 41 and 42 that are electrically connected to the respective circuits of pumps 33 and 34. Due to the mass and consequent inertia of body 36 of control switch and the balanced effects of springs 37 and 38, movable contact 40 will not be shifted axially sufficiently under the influence of normal vibratory movement of connecting rod 19 to make contact with either stationary contact 41 or 42 to close the circuit of either pump. It is only when a change in the pressure diiference within and outside the enclosed space of housing 10 sufiicient to displace diaphragm 11 abnormally occurs that switch 35 will be fully operated to close the appropriate pump circuit to reestablish normal pressure conditions and thereby return diaphragm ill to properly centered position.

It should be apparent that, while I have used two air pumps, it would be equally feasible to use one pump controlled by a valve arrangement (not shown) which would change the direction of air flow as dictated by the diaphragm position.

Aithough it was stated earlier herein that avoidance of the use of horns is contemplated, there would appear to be no difference in the principle of this invention whether or not a horn is used with the speaker. A small horn will increase elficiency and permit the use of a smaller diaphragm and smaller amplitude for a given sound output. These benefits are in addition to the advantages of a low resonant frequency in the presence of an enclosed air volume which this invention permits. The addition of a very large horn would actually lessen the need for the present invention, because it would permit the diaphragm to be small enough so that the enclosed air volume need not be impracticably large. Moreover, a very efiicient horn would add mass loading to the diaphragm and thereby cause the resonant frequency to be low even in the presence of considerable stiffness. A highly etlicient horn would also provide damping to an extent which would obscure resonance. However, as previously explained, a horn of such efficiency at 30 cycles is extremely and prohibitively large. For example, by classical formulae a horn whose lower cut-off frequency is 30 cycles has a mouth ten feet square, or one of equivalent area. That is why the employment of any horn is not contemplated in the structural embodiment of my present invention.

Although a toggle mechanism appears at this time to be the most simple and readily adaptable means for applyiug negative stilfness to a loudspeaker diaphragm to counteract positive stiffness, there may be equivalent means for this purpose available for substitution without departing from the spirit of the present invention. Even the particular toggle structure disclosed as an illustrative example in Figures 1 and 2 may be modified within that spirit, as shown in Figures 4 to 6.

Figure 4 discloses a simplified embodiment of the toggle principle in which a unitary U-shaped spring 43 of flat strip metal supplants the two coil springs 303tl and levers 27-27 of the Figures 1 and 2 embodiment. The medial portion of this spring 43 may be fixedly secured as shown to the central hub portion of frame 17 as a means of rigid support and preferably is apertured to slidingly accommodate the forward end portion of connecting rod 19a. The opposed resilient arms 4343 of spring 43 are provided terminally with indentations 44-44 to engage the outer ends of struts 21a-21a in the usual manner, the inner ends of the latter being engaged Within transverse grooves 4545 in the enlarged portion 24a of rod 19a. In accordance with the principle of the embodiment of toggle mechanism disclosed in Figures 1 and 2, the arms 43'43 of spring 43 should be formed so that they will meet to form a triangular structure with the medial portion of the latter as the third arm when disassembled and in untensioned condition, so that these arms will be placed under the desired tension when spread apart for engagement with the outer ends of toggle struts 21a-21a.

In Figure 5, there is illustrated another modification in which a U-shaped spring 46 similar to that shown in Figure 4 is used. In this instance, however, the ends of the resilient arms 4646' of spring 46 have indentations 4'4--== 44 fof engagementwitlrth'emutefiend portions of a -u-nitary double strut memb'e'r'47 'wh'osmflexi ta-medial portion-is affixed, as by'soldering,lto connecting rod 1'9bI The double strut member thus flexes in the middle so that' the interconnected outer halves act as if they-Were the individual struts used in the previously described embodi ments (Figures 1, 2 and 4). V

Figure 6 discloses a still further modification'in which frame 17 is provided with a tubular central hub portion 1 7' arranged coaxial with connecting rod 19c and of sufficient diameter to enclose'and guard the negative stiffness unit; There also are'two of the'U-shaped springs 48 and 49 arranged with their medial p'ortiens back-to-back and affixed to a cross Wall 50of"-fram'ehub portion 17". Thereare two double strut members SI-and 52, respectively, similar to those shewnin Figure whose outer ends are engaged withth'eindentations44 44" in the free 'endsof the respective 'sp'ring iarms and whose-flexible medial portions are aflixed as by' soldering to connecting rod'19c'. This double unit gives supporttob'oth ends of rod'19c' and is 'especiall'y suitable for use inanapplication wherein the diaphragm or equivalent vibratory element" has unusually largemas's and amplitude of movement.

Another useful application'o'f the negative stiffnessiunit which occurs to me n'owis in connection withthe Sylphon bellowsdiaphragm of instruments for indicating pressure changesor' functions of pressure changes which may have some especial significance. For example, in'a typical airspeed indicator for aircraft the ratio of the air pressureir'i'a p'i'tottube arranged parallel to the direction of flight (major axis of the aircraft) to the pressure in a static tube arranged'at rightangles to the pitot tube varies asa functio'nof airspeed. Therefore, the instrument incl n'desniechanism to translate changes in the pressure difference into airspeed readings on an indicator scale.

In Figure 7, an airspeed indicatorof the pitot-static tube {type is illustrated. In this instrument, adiaphragm SS- clo'ses the inner free end of'the conventionalS'ylphon bellows 54, which has the characteristic corrugated peripheral wall 55 of'resi'lient metal to permit axial extension and construction of the bellows under'the influence of internal pressure changes. Syl'phon bellows5'4 is supported inside a fluid-tight casingi'56 by rigid attachment to said casing of the end of the bellows'oppositefto diaphragm 53. The associated pitot tube57 and static tube Share communicably connected, respectively, to the interior of Sylphon'b'ellows'54 and the interior 'of'casi'ng 56.

In the operation of this'i'nstr'ument in flight, the difference in air pressure within. pitot tube'57 and static tube 5 8, which is a function of airspeed, causes'displacement ofdiaphragrn 53 axially from'its normal p'ositionof rest inwhich the corrugations of wall 55 are retracted into unten'sione'd condition. In this instance, diaphragm 53 issubject'e'd to positive mounting stiifnessinste'ad of air stiffness, because it is opposed in its operational displacement by the resistance to axial extension ofthe resilient corrugated wall 55' that serves to'mount thediaphragm for its intended displacement.

In accordance with the fundamental principle of my invention, the sensitiveness of the airspeed indicator is d'esirably increasedby applying. negative stiffness to diaphragm 53: in the samemanner that'has been done in the case of the loudspeaker. Therefore, diaphragm 53 is connected by rod59 to the medial flexible portion of a unitary type toggle strutmember 60of a-neg'ative stiffness unit 61 of the same form as that represented in Fig. 5. The U-shaped spring 62 of unit 61 is medially affixed to frame member 63 supported rigidly by housing 56. The negative stifiness unit represented 'in Fig. 7 operates in the manner previously describedto negate theefiects of positive mounting stiffness on diaphragm 53.

In the airspeed indicator application of the present invention, airspeed functionally related to the pressure dilference in pitot tube 57 and static tube 58 is indicated 10 on appropriately calibrated scale 64 by a needle se ene ably operated bydiaphragni 53, as b'y direct'articuIated connection to rod '59. i

As used in the specification and claims, the term dia phragm is to be understoodto mean any diaphragm, membrane, or equivalent movable structural element of an instrument or device, whose operational displacement is opposed by positive stiffness of its mounting means or that produced by compression of afluid volume or other elastic medium by that displacement.

It is to be understood also that, although an elongated connecting rod has been chosenjfor the purpose of illus-' tration as means to transmit axial motion between the toggle struts of the negative stiffness unit and the diaphragm subjected to positive stiffness in each of the illustrative applications of the invention, any other equivalent means for transmitting motion may be employed within the spirit of the invention. In fact, it is entirely practicable to pivotally engage the inner ends 'of the toggle struts directly with the diaphragm, although this arrangement has not been shown.

It will be understood, too, that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. In an instrumental device, the combination of: a diaphragm; resilient means opposing the operational displacement of said diaphragm and subjecting it to stiffness; means to cause said operational displacement of the dia phragm; and means to counteract the effect of stiffness on the diaphragm during'opera'tional displacement and I to increase said displacement, said means comprising force production means and meansto' transmit force from said force production means'to the diaphragm in proportion to its operational displacement from its posi-' tion of equilibrium, said force being in the same direction as said displacement.

2. In an instrumental device, the combination of: a stationary frame; a diaphragm mounted foroperational displacement in said frame; resilient means opposing the operational displacementof said diaphragm and subjecting it to stiffness; means to cause said operational dis placement of the diaphragm; and means to counteract the effect of stiffness on the diaphragm"duringoperational displacement and to increase said displacement, said means being in the form of a toggle mechanism including a connecting rod attached to the diaphragm in axially extending relation thereto and adapted to reciprocate longitudinally in synchronism with operational displacement of said diaphragm, toggle strut means consisting; of at least one radially arranged strut member having.

pivotal engagement between its inner end portion and the connecting rod and being adapted to be in deadcenter alignment and unstable equilibrium when the diaphragm is in undisplaced condition, and spring-loading means applied to the outer end of the toggle strut member to exert radially inward force thereto in' relation to the connecting rod axis whereby up'on displacement of the diaphragm axially an axial force component will be applied to the latter in the direction of displacement.

3. Inan instrumental device, the invention as defined in 'claim 2, wherein theoretically perfect'forcedisplacemerit linearity is achieved in use'of the toggle mechanism by such construction and arrangement of the spring. loading means that, the spring displacement measured at the outer end of the'strut member is equal to the effective strut length, whereby the axial force component will be exactly proportional to the axial displacement.

4. In an instrumental device, the invention as'defined in claim 2', whereinthetoggle mechanism include-sat least one lever arranged to extend lengthwise of the connecting rod axis and in the axial plane of the toggle strut means and having an end portion thereof in pivotal en- 1 1 gagement with the outer end of the strut member; wherein the opposite end portion of the lever is in pivotal engagement with the stationary frame; and'wherein the spring loading means is arranged to press radially inward against the medial portions of said lever.

5. In an instrumental device, the invention as defined in claim 2, wherein the toggle mechanism includes a pair of diametrically opposed strut members and at least one unitary U-shaped spring medially affixed to the stationary frame with its resilient arms extending lengthwise of the connecting rod in the plane of the toggle strut members and having the end portions of said arms pivotaliy engaged with the outer ends of said strut members.

6. In an instrumental device, the invention as defined in claim 5, wherein the toggle strut means is in the form of an integral structure having a flexible medial portion afiixed to the connecting rod in a manner to afford pivotal engagement therewith due to said flexibility.

7. In an instrumental device,tl1e invention as defined in claim 6, wherein there are two springs of U-shape arranged back to back with their medial portions affixed to the stationary frame, and two axially spaced strut members terminally engaged with the resilient arms of the respective springs.

8. In an instrumental device, the invention as defined in claim 7, wherein a tubular guard member supported by the stationary frame is arranged concentric to the connecting rod and in enclosing relation to the spring and strut means.

9. In an instrumental device, the invention as defined in claim 2, wherein stop means are provided on the connecting rod for displacement limiting contact with the stationary frame, the arrangement of said stop means with respect to said frame and the spacing therebetween being such that limiting contact will occur upon displacement of the diaphragm in either direction to a predetermined degree.

10. In a loundspeaker, the combination of: structural means enclosing an air space; a diaphragm mounted with one side thereof facing said air space in a position to radiate sound waves into said space, whereby the diaphragm is incidentally subjected to stiffness in operation produced by the forces of compression and expansion of the air in said space caused by diaphragm action; means to drive the diaphragm; and means to counteract the effect of stiffness on the diaphragm during operational displacement and to increase said displacement, said means comprising force production means and means to transmit force from said production means to the diaphragm proportionate to the displacement of the latter and in the same directions.

.11. In a loudspeaker, the invention as defined in claim with the addition of means responsive to operational displacement of the diaphragm for actuating the transmission of force to the latter from the force production means.

12. In a loudspeaker, the invention as defined in claim 10, wherein the means to counteract the effect of stitfness on the diaphragm is in the form of a toggle mechanism including a connecting rod attached to the diaphragm for longitudinal movement in synchronisrn therewith, a pair of toggle struts arranged to project radially from the connecting rod in diametrically opposite directions and having their inn r ends pivotally engaged with said rod and being adapted to be in dead-center alignment and unstable equilibrium when the diaphragm is in midposition, and spring-loading means applied to the outer ends of the toggle struts to exert radially inward force thereon in relation to the connecting rod axis.

13. In a loudspeaker, the invention as defined in claim 12, to which is added stop means to limit axial displacement of the connecting rod in opposite directions from the medial position thereof corresponding to the deadcenter relation of the toggle links.

14. In a loudspeaker, the combination of: structural means enclosing an air space and including a stationary frame; a diaphrgam mounted in the frame with one side thereof facing the air space in a position to radiate sound waves into said space, whereby the diaphragm is incidentally subjected to stiffness during vibrational displacement produced by the forces of compression and expansion of the air in said space caused by said displacement; means to drive the diaphragm; means to counteract the effect of stiffness on the diaphragm during vibrational displacement, said means being in the form of a toggle mechanism including a connecting rod attached to the diaphragm in coaxial relation thereto and adapted to reciprocate in direct response to vibrational displacement thereof, at least one toggle strut arranged to project radially from the connecting rod with its inner end pivotally engaged with said rod and being adapted to be in dead-center alignment and unstable equilibrium when the diaphragm is in mid-position, and spring-loading means applied to the outer end of the toggle strut to exert radially inward force thereon with an axial component when the diaphragm is displaced in either direction from mid-position; and means to regulate the internal pressure of the enclosed air space in normal balanced relation to outside pressure automatically to prevent excessive displacement of the diaphragm and locking of the same in excessively displaced position under the thrust of the toggle mechanism when an unbalanced pressure difference inside and outside occurs, said means including pumping means communicating with said air space and adapted in one operational condition to expel air from said space and in another condition to introduce air thereto, and means responsive to excessive displacement of the diaphragm in either direction from mid-position to actuate the pumping means in the appropriate operational condition to restore the pressure balance to normal and thereby return the diaphragm to mid-position.

15. In a loudspeaker, the invention as defined in claim 14, wherein the pumping means is electrically operated and has two selective electrical circuits therefor; and wherein switch means responsive to displacement of the diaphragm to a predetermined degree in either direction due to a volume change inside the enclosed air space is adapted to close the appropriate circuit automatically to produce an operative condition of the pumping means that will restore the diaphragm to its mid-portion.

16. In a loudspeaker, the invention as defined in claim 14, wherein the pumping means is electrically operated and has two normally open electrical circuits connected to opposite poles of a source of current, switch means comprising stationary insulated contacts connected to the respective circuits, and a movable contact connected to the source of current and operated by the connecting rod of the negative stifiness means and arranged to be in open condition when the diaphragm is in mid-position and in normally displaced positions but to make contact with the appropriate stationary contact when the diaphragm becomes displaced to a predetermined abnormal degree in either direction.

17. In a loudspeaker, the invention as defined in claim 16, wherein the movable contact of the pressure regulating means is spring balancedin open condition and of such mass that inertia of the same will prevent switch closing motion under the influence of the normal vibrational displacement of the diaphragm alone.

References Cited in the file of this patent UNITED STATES PATENTS 1,606,571 Heising Nov. 9, 1926 2,141,208 Eaves Dec. 27, 1938 2,655,566 Pittinger Oct. 13, 1953 2,661,412 Dreyfus Dec. 1, 1953 

